Population Size and Extinctionpeople.cst.cmich.edu/swans1bj/620 lect 02.pdf · Extinction Small populations are more likely to go extinct than large populations Genetics ... which

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“Science is simplycommon sense at itsbest that is, rigidlyaccurate inobservation, andmerciless to fallacyin logic.”

Thomas Henry Huxley

Terminology Locus Allele Gene Chromosome Homozygote Heterozygote Polymorphic Monomorphic

Terminology Polygenic Epistatic Genotype Phenotype Quantitative trait Neutral Marker

Population Size andExtinction

Small populations aremore likely to goextinct than largepopulations Genetics Demographic



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Genetic Variation andPopulation Size Black-footed rock wallaby

Genetic Variation andPopulation Size

Genetic Variation &Population Size

Larger populationsizes have moreheterozygosity.

Halocarpus bidwilliPicoides borealis

Genetic drift andheterozygosity

Genetic drift results in a gradual loss ofgenetic diversity

Follow an individual locus and allelefrequency will drift until one allelebecomes fixed

Genetic drift

107 populations of Drosophila16 individuals per population

Genetic Variation& Fitness

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Inbreeding &Population Size Inbreeding

Inbreeding is related to populationstructure

Consider (a) one large panmictic population (b) lots of small, discrete populations

Identity by descent

assume every allele in initial population is differentIBD = probability of this individual carryingtwo copies of the same allele

=

Probabilities

Probability of event A happening ANDevent B happening = PA x PB

Probability of event A happening ORevent B happening = PA + PB (if A & B mutually exclusive)

Identity bydescent

• Probability of black allelebeing passed down left handside = 0.5 x 0.5 x 0.5 = 1/8

• Probability of black allelebeing passed down right handside = 0.5 x 0.5 x 0.5 = 1/8

• Probability of individual atbottom getting two blackalleles = 1/64

• Same argument for yellowallele = 1/64

• Probability of getting 2copies of black or 2 copies ofyellow = 1/64 + 1/64 = 1/32

Inbreeding in finitepopulations

Previous examplesshowed inbreeding inknown pedigrees throughIBD

What about in finitepopulations?

In a finite population,there is some probabilityof mating with a relative

This probability increaseswith subsequentgenerations

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Assume a population size of N, therefore 2N alleles inpopulation

Imagine eggsand spermreleasedrandomly intoenvironment(e.g. sea)


Assume a population size of N, therefore 2N alleles inpopulation

Imagine eggs and sperm released randomly intoenvironment (e.g. sea)

What is the probability of 2 gametes drawn randomlyhaving the same allele?

Why?2N allelesgen 0

gen 1

probability = 1/(2N)

You already have one allele withwhich you started, thus you only needto pick 1 additional allele


Therefore, after 1generation the level ofinbreeding is F1 = 1/2N

After t generations theprobability is

Why?

gen t-1

gen t

1/(2N) 1 - 1/(2N) Probability ofpicking 2nd allele

Probability ofpicking 2nd allele &it is already inbred

More generally

GeneticDrift

Ft = Probability thatany 2 alleles drawnrandomly from thepopulation are identicalby descent

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Measures of Inbreeding1. Inbreeding coefficient F

0.5Selfing (self-fertilization)0.0625First cousins0.125Half sibs (half brother to half sister)0.25Brother to sister, mother to so, etc

0Unrelated

OffspringF

Parents

Extinction Factors

Extinction Vortex Inbred individuals are less

fit then outbred individuals Deleterious recessive

mutations tend to becomeapparent in inbredindividuals

Inbred individuals mayhave lower reproductivesuccess, lowersurvivorship or lowerresistance to parasites

Inbreeding and Survival

Inbreeding increases juvenile mortality


Inbreeding & Survival Inbreeding and Survival

Inbreeding doesn’t influence allspecies the same or does it?

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Inbreeding doesn’t influence allspecies the same or does it?

Inbreeding and conservationgenetics

Inbreeding has greatest effects in small populationsor populations that have suffered a bottleneck

Loss of genetic diversity inthe cheetahsevere bottleneck in its past

Inbreeding example 1Keller et al Nature 372:356-357

Sparrows on Mandarte Is., BC Inbreeding associated with poor survivorship Paternities determined by genetic fingerprinting Population crash Survivors less inbred than those that die

0.0156 (n=3)0.0025 (n=7)After crash0.0363 (n=132)0.0189 (n=74)Before crashJuvenilesAdults

inbreeding coefficient

Distribution of inbreeding coefficients forsurvivors/non-survivors

survivornon-survivor

Inbreeding example 2:Soay sheep on St. Kilda(Coltman et al Evolution 53:1259-1267)

Soay sheep subject toparasitism by intestinalnematodes

Use markers distributedthrough the genome

Heterozygous individualsappear best able to resistinfection and to survive

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Heterozygosity & parasitism

filled, low-densityyears

open, high-densityyears

(FEC = fecal eggcount of strongylenematodes)

Heterozygosity & survivorship

filled, low-densityyears

open, high-densityyears

(survivorship oflambs over winter)

Factors acting againstgenetic drift

No matter how big a population, geneticdrift will eventually drive all loci tohomozygosity (i.e. all alleles at all loci inall indvs. identical by descent)

Mutation and migration counters geneticdrift

Analysis of effect of these processesidentical.

“It requires a veryunusual mind toundertake the analysisof the obvious.”Alfred North Whitehead

“We see only what weknow.”

Johann Wolfgangvon Goethe

EVOLUTIONARY PROCESSESAND PATTERNS

Theories of species creationsand diversity prior toevolutionary theory

Theory of Special Creation Species are unchanged

through time and areindependent of one another

All species were createdindependently by “…theTrinity on the October 26th4004 B.C. at 9:00 in themorning” Archbishop JamesUssher 1664.


Theories of species creations and diversity priorto evolutionary theory

Theory of Spontaneous Generation New organisms (species) may suddenly appear

wherever conditions are suitable Some new life-forms arise spontaneously from

streams, soils, totting meat, and other nonlivingmaterials; not all live arises directly from livingorganisms

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1620’s

VanHelmont’s evidence forspontaneous generation

“…for if you press a piece of underware soiled with sweattogether with some wheat in an open mouth jar, afterabout 21 days the odor changes and the ferment comingout of the underwear and penetrating through the husksof the wheat, changes the wheat into mice. But what ismore remarkable is that mice of both sexes emerge (fromthe wheat) and these mice successfully reproduce withmice born naturally from parents…But what is even moreremarkable is that the mice which came out were notsmall mice… but fully grown.”


Theories of species creationsand diversity prior toevolutionary theory

Prior to Darwin and Wallace -Lamarck New simple life forms arise by

spontaneous generation andchange over time into morecomplex life forms

Individuals change in response totheir environment and thechanges are passed to the nextgeneration.


Theories of Evolutionary Darwin and Wallace Species are related to one another, and they

change over time, thus species existing todayhave descended, with modifications, from otherpreexisting species.

Natural selection acts on individuals; individualswith certain favorable characteristics will producemore offspring.

Darwinian Selection All natural selection results in evolution, but not

all evolution is the product of natural selection. What is evolution?

What is natural selection?

What is an adaptation?

Darwinian Selection All natural selection results in evolution, but not all evolution is

the product of natural selection. What is evolution?

Evolution is the change in allele frequencies (or traits) over time. Selection

• Natural• Sexual

Genetic drift Mutation Migration

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Darwinian Selection All natural selection results in evolution, but not all

evolution is the product of natural selection. What is evolution? Evolution is the change in allele frequencies (or traits)

over time. What is natural selection?

What is an adaptation?



over time. What is natural selection? Natural selection is the differential reproductive success

resulting from an adaptation. What is an adaptation?



over time. What is natural selection? Natural selection is the differential reproductive success

resulting from an adaptation. What is an adaptation? An allele (or trait) which provides superior reproductive

success to an individual possessing it relative to anindividual with a different allele (or trait).

Darwinian Selection

Sum it all upNatural selection for anadaptation results inevolution.

What can selection do to atrait?

If a trait is underselection, what are thepossible results?

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Chimp Lucy Human

Outlet

Midcavity

Inlet

Female Mimicrya - unpaired male (m) assumes female coloration and posture while approaching the female (f) as the consort male (c) fights with an approaching male (m2)

m2

b - the female accepts a mating by the female mimic while the consort continues fighting with the approachingmale

c - The consort male allow the female mimic to finishmating without interruption, even when he is notdistracted by the other male

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Insects, spiders, nectar

Tools use to get insects

Leaves and fruit

Ticks off of iguanas etc.

Seeds

Darwinian Selection

Is there variation about a trait?

Darwinian Selection

Is the variation heritable and not the result of maternal effects?

Darwinian Selection

Is there an excess of individualsso that only some animals liveto reproduce?

Is there an excess of individualsso that only some animals liveto reproduce?

Darwinian Selection

Is reproduction nonrandom?

The drought of 1977 eliminatedseed set by most of the plantsproducing small soft seeds.Tribulus cistoides seeds are largeand hard and became the dominantfood item. Only large birds withdeep beaks could defend resourcesand access the resources

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Darwinian Selection

Is reproduction nonrandom?

Darwinian Selection

Did evolution occur?

The El Niño of 1983 produced1359 mm of rain and lavish seedset by the small soft seeded plants.Birds with shallow beaks harvestthese seeds more efficiently andthus reproduced better than birdswith deep beaks, undoing theselection shown here. Fluctuatingenvironmental conditions maintainboth phenotypes.

At what level does selectionoccur?

Gene … for the good of the gene.

Individual … for the good of the individual.

Group … for the good of the group.

Population … for the good of the population.

Species … for the good of the species.

Darwinian Selection

THEINDIVIDUAL

Darwinian Selection

NOT THEGROUP!!!*

*In silico (computer simulations) do show specific conditions where group selection can work but This has never been seen in nature

Darwinian Selection

NEVERTHE

SPECIES!!!

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Darwinian Selection Darwinian Selection


Darwinian Selection

P.S.Darwinian Selection

THERE ISNO

ALTRUISM!

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Darwinian Selection

Why does it appear that some animalshelp others at a cost to themselves?

Darwinian Selection Hamilton’s inclusive fitness

Direct Fitness Personal reproduction

Indirect Fitness Relatives reproduction

Br - C > 0 What are the units of B and C?

Fitness must be in addition tothat which you would havegotten from the relativeregardless of the behavior!

Darwinian Selection

Who will you save from afire? Brother Cousin 1 Cousin 2 Father Friend Mother Sister Spouse

Darwinian Selection

Who will you save from a fire? Brother - Age 21 Cousin 1 - Age 19 Cousin 2 - Age 20 Father - Age 65 Friend - Age 23 Mother - Age 63 Sister - Age 22 Spouse - Age 25


Who will you save froma fire? Brother - Age 21

Cousin M - Age 23 Father - Age 65

Mother - Age 63 Sister - Age 22

Cousin F - Age 19 Spouse - Age 25

Cousin F - Age 20

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Darwinian Selection





Cousin F - Age 20

Darwinian Selection





Cousin F - Age 20

Darwinian Selection





Cousin F - Age 20

Darwinian Selection





Cousin F - Age 20

Darwinian Selection

The Modern Synthesis Combines genetics, geology, and natural

selection Gradual evolution results from small genetic

changes acted upon by natural selection Speciation and macroevolution can be

explained by microevolution given the age ofthe earth

Darwinian Selection

Modern Synthesis - Darwin’s postulates restated Mutation is the ultimate source of all variation,

segregation and independent assortment createnovel combinations of alleles

Alleles are passed between generations More offspring are produced than can survive Individuals with the most advantageous allelic

combination for the current situation produce moreoffspring.

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God does not playdice with theuniverse.

Albert Einstein

Who are you totell God whatto do?

Neils Bohr

Not only does Godplay dice, but hethrows them wherewe cannot seethem.

Stephen Hawkings

Happy is he whogets to knowthe reason forthings.

Virgil

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Population Size and Extinctionpeople.cst.cmich.edu/swans1bj/620 lect 02.pdf · Extinction Small populations are more likely to go extinct than large populations Genetics ... which